Monitoring and analyzing turbine blade health is a vital ingredient to ensuring proper performance of jet engines. As turbine blades rotate, they tend to vibrate. These vibrations induce stresses in the blades. These stresses can eventually lead to a failure condition known as high-cycle fatigue failure (HCF). This type of failure may initially manifest itself as small cracks. Over time, the crack propagates, and eventually leads to catastrophic failure of the rotating machinery. Real-time knowledge of turbine blade vibration is critical to predicting and preventing such failures.
While HCF failure is perhaps the most insidious failure mode, turbomachinery is also susceptible to more rapid forms of failure. For example, foreign object damage (FOD), where a jet engine ingests debris such as pebbles, has led to a number high profile incidents. Engines can also be damaged by out of balance conditions and accessory failures. These are among the many turbomachinery failure modes that can be detected and possibly prevented through blade vibration monitoring.
Blade tip sensors are typically used to provide continuous monitoring of turbine blades. Blade tip sensors are typically embedded in a jet engine case and used to measure blade tip clearance from walls of the engine, in addition to blade vibration. Recent developments have seen the introduction of multiple non-contacting sensors, such as, but not limited to, eddy current sensor (ECS), into turbomachinery. An ECS employs an active magnetic field to monitor each blade as it passes the sensor. An ECS generates an electrical signal proportional to the distance of a blade from the sensor. Existing algorithms extract two pieces of information from the signature of a non-contacting sensor, signal magnitude and signal zero-crossing time, which are used to estimate vibrational parameters over the course of multiple revolutions. These techniques fail to exploit a majority of the information contained in the non-contacting sensor signal.
One example of current ECS use is in the F-35 Joint Strike Fighter engine. Unfortunately, the eddy current sensors, and other non-contact sensors, are used only to monitor blade tip clearance, which for eddy current sensors is derived by simply taking the maximum value of each ECS pulse. All other data in the ECS pulse are discarded. Most current NSMS (Non-Contact Stress Measurement System) research is primarily focused on a technique known as tip timing. Tip timing systems generally employ special hardware to accurately measure the time at which an ECS pulse passes through zero volts. Since vibrating blades arrive at the sensor at slightly different times, the time of zero crossing is also slightly different, depending on the vibration characteristics. These small timing differences allow blade vibration to be inferred.
Tip timing has a number of deficiencies. It is an inherently aliased approach, since effectively only a single sample per revolution is taken from each blade. Thus, resolving blade vibration often requires multiple sensors. Unfortunately, it is undesirable to have multiple holes within an engine housing for purposes of supporting the eddy current sensors. In addition, tip timing fails to exploit the majority of the data available in an ECS waveform.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.